That's dirt cheap. Work out your volume of concrete at about 150mm thick and 6m3 of ready mixed concrete is well over £600 these days. In fact I'm suspicious that £550 is too cheap TBH, without doing the sums.

Done the sums!
3m3 of concrete roughly but you pay for the 'uncarried' too on part loads, 6m3 being a full load. Then you need shuttering, reinforcing, lining, tools....
For £550 I seriously wouldn't bother DIY.

cheers for the advice , its gonna be a vespa/cycle store with space for a bench and bike work stand , iam fed up of waiting for a fine day to work on the bike and the vespa doent like being outside over winter , gonna get some leccy in there also

Won't need building regs for one that size. Also I only make it 1.5m3 of concrete. If you can get the mixer in close it won't take long to pour. Less than half a day for you and a mate plus £150 for the concrete.

Nickjb yer spot on with the amount, at 150mm thick 1.5cube if you had it delivered it would cost about £240.00
Then youve got 2 sheets of reinforcing mesh £40.00 and a roll of 2000 grade
Visqueen £25.00 and a couple of sheets of shutter ply £36.00. Then a days labour for 2 men £300.00,
£616.00 + vat £738.00 so at £550.00 thats dirt cheap.

DIY will take 4 to 6 hours for 3 of you, one on the mixer and two working the barrow and mixed muck. Personal experience would recommend that you hire or borrow the largest mixer you can. A barrow mixer will take ages the larger ones produce about 2 barrows of mix per load. Bear in mind the diesel donkey engines are loud so warn the neighbours.

DIY materials for 1.5 cubic m of concrete would be 3 bulk bags of aggregate and 18 bags of cement. Don't underestimate how much effort this is going to take to mix - there's over 3 tons of material here, and that's about 30 mixes in a small Belle-style mixer. 6 minutes per mix would be good going, so 3 hours of non-stop graft.

Pp where do you buy your concrete from, a man with a mask and gun?
Fwiw 1.5m3 wil be four bulk bags at a minimum of £28 each and 20 bags of op, at £3.50 ish. So materials alone are nearly £200. I can get 1.5 delivered on a pay what you lay lorry for that price, and for that it'll be a better mix and more consistent mix wise.
The lads that have quoted 550 aren't far wrong and in real terms that isn't way too cheap.
I can shutter a base like that in 15 mins.
Also if you do decide to go DIY have you "handled" ready mix before? You'll be paying waiting time on anything over 20 mins for that sort of quantity.